Numerical studies on time variations of the troposphere-stratosphere coupled system

Interannual variation is a year-to-year variation which is defined as a deviation from the climatological annual cycle of a meteorological quantity. It can be caused by a variation of an external forcing of the atmospheric circulation system, or can be generated internally within the system. On the other hand, intraseasonal variation is a low-frequency variation within a season, and it is basically considered to be a result of internal processes which may exist even under constant external conditions. In this article, some observational facts on the intraseasonal and interannual variations of the polar stratosphere are presented, and the use of a hierarchy of numerical models to understand the stratospheric variations is reviewed systematically. Numerical models, can be roughly divided into three classes based on their complexity; simple low-order models, medium mechanistic circulation models, and complex general circulation models. In order to understand the stratospheric variations, a hierarchy of stratosphere-only models have been used under an assumption of "slave stratospheric-variations" or "independent stratospheric-variations". Parameter sweep experiment, in which many trials of computations in parameter space are done by sweeping the value of a control parameter, is a powerful method to understand complex behavior in the models. Recently, however, the importance of the coupled variability of the troposphere and the stratosphere was pointed out for the intraseasonal and interannual variations. In addition to numerical studies with simple or complex models, we have done some parameter sweep experiments with three-dimensional mechanistic circulation models to understand the troposphere-stratosphere coupled variability. All the effects of external forcings that might cause interannual variations can be excluded in the numerical experiments to focus only on internally generated variations within the coupled system. The obtained intraseasonal and interannual variations have some similar characteristics of the real atmosphere in some realistic parameter ranges. Roles of the interannual variations of the external forcings are discussed, which might be significant even if the amplitude is small.